Mailing systems for printing postage indicia on envelopes and other forms of mail pieces have long been well known and have enjoyed considerable commercial success. There are many different types of mailing systems, ranging from relatively small units that handle only one mail piece at a time, to large, multi-functional units that can process hundreds of mail pieces per hour in a continuous stream operation. The larger mailing systems often include different modules that automate the processes of producing mail pieces, each of which performs a different task on the mail piece. The mail piece is conveyed downstream utilizing a transport mechanism, such as rollers or a belt, to each of the modules. Such modules could include, for example, a singulating module, i.e., separating a stack of mail pieces such that the mail pieces are conveyed one at a time along the transport path, a moistening/sealing module, i.e., wetting and closing the glued flap of an envelope, a weighing module, and a metering module, i.e., applying evidence of postage to the mail piece. The exact configuration of the mailing system is, of course, particular to the needs of the user.
Typically, a control device, such as, for example, a microprocessor, performs user interface and control functions for the mailing system. Specifically, the control device provides all user interfaces, executes control of the mailing system and print operations, calculates postage for debit based upon rate tables, provides the conduit for the Postal Security Device (PSD) to transfer information defining postage indicia or a digital postage mark (DPM) to the printer, operates with peripherals for accounting, printing and weighing, and conducts communications with a data center for postage funds refill, software download, rates download, and market-oriented data capture. The control device, in conjunction with an embedded PSD, constitutes the system meter that, for example, satisfies U.S. information-based indicia program (IBIP) meter requirements and other international postal regulations regarding meters. The United States Postal Service (USPS) initiated the Information-Based Indicia Program (IBIP) to enhance the security of postage metering by supporting new methods of applying postage to mail. The USPS has published draft specifications for the IBIP. The requirements for a closed system are defined in the “Performance Criteria for Information-Based Indicia and Security Architecture for Closed IBI Postage Metering System (PCIBI-C),” dated Jan. 12, 1999. A closed system is a system whose basic components are dedicated to the production of information-based indicia and related functions, similar to an existing, traditional postage meter. A closed system, which may be a proprietary device used alone or in conjunction with other closely related, specialized equipment, includes the indicia print mechanism.
The PCIBI-C specification defines the requirements for the indicium to be applied to mail produced by closed systems. The indicium consists of a two-dimensional (2D) barcode and certain human-readable information. Some of the data contained in the barcode includes, for example, the PSD manufacturer identification, PSD model identification, PSD serial number, values for the ascending register (the total monetary value of all indicia ever produced by the PSD) and descending register (the postage value remaining on the PSD) of the PSD at the time of printing, postage amount, and date of mailing. In addition, a cryptographic digital signature is required to be created by the PSD for each mail piece and placed in the digital signature field of the barcode. Several types of digital signature algorithms are supported by the IBIP, including, for example, the Digital Signature Algorithm (DSA), the Rivest Shamir Adleman (RSA) Algorithm, and the Elliptic Curve Digital Signature Algorithm (ECDSA). Each of the supported digital signature algorithms implements a “public key” cryptographic algorithm for the digital signature function. Public-key cryptosystems allow two parties to exchange private and authenticated messages without requiring that they first have shared a private (symmetric) key in a secure fashion. A public-key cryptosystem utilizes a unique pair of keys: a private key that is a secret and a public key that is widely known and can be obtained and used by any party without restrictions. This pair of keys has two important properties: (1) the private key cannot be deduced from knowledge of the public key and the message, and (2) the two keys are complementary, i.e., a message encrypted with one key of the pair can be decrypted only with the other (complementary) key of the pair. As described in the PCIBI-C specification, the PSD internally derives the private/public key pair. Both the public and private key are stored in nonvolatile memory in the PSD. The public key is then provided to a certificate authority, which generates a certificate for the public key that verifies the authenticity of the public key. The certificate is returned to the PSD, which compares the stored public key with the public key included in the certificate. If the comparison is successful, the certificate for the public key is stored by the PSD.
The PSD then utilizes the private key to cryptographically sign indicia, which evidences payment of postage, produced by the PSD. The digital signature allows the postal service to authenticate each indicium, and provides assurance that proper accounting has been performed and payment has been made for delivery of a mail piece. To authenticate each indicium, the postal service utilizes the public key, in conjunction with the certificate for the public key, to verify the digital signature of the indicium. Accordingly, the postal service requires access to the appropriate public key corresponding to the signature, along with the certificate for the public key. One way to provide suitable access would be to include the public key and corresponding certificate on the face of each mail piece along with the indicium. Because of the size and complexity of the public key and certificate, this is difficult and costly to do. Another way to provide suitable access is by providing suitable key management, in which the manufacturer of the PSDs provides the public keys and certificates for its PSDs to the postal service. This can be performed, for example, using electronic or physical means. The postal service must then maintain a suitable repository of each of the public keys for use in verifying indicia (i.e., when the public keys must be retrieved from the repository). Each of these, however, adds significant costs for both the PSD manufacturer and postal service with respect to record keeping and infrastructure to support such key management. Another problem with such systems is lack of, or expense of maintaining, a managed certificate or public key revocation system. The PSD manufacturer will, from time to time, revoke a current set of keys being used (due to, for example, a possible security breach). Ideally, when verifying an indicium the postal service will ensure that the key pair used for the indicium has not been revoked. This, however, also adds additional costs to the verification process, and in many cases the revocation check is not performed.
Thus, there exists a need for methods and systems for authenticating indicia that do not conventional and expensive require key management systems, and in which revocation of key pairs is easily performed without adding costs to the authentication process.